1. Field of the Invention
The invention relates to use of structures composed of diamond with metals and/or their compounds, for the formation of both static and dynamic X-ray anodes.
2. Description of the Prior Art
High power X-ray sources are desirable for applications such as X-ray lithography, X-ray tomography, X-ray transmission and interference microscopy and high resolution X-ray Photoelectron Spectroscopy (XPS). The intensity of X-ray tube sources are currently limited largely by the thermal conductivity and temperature of melting/sublimation of anode materials and formation of high density electron beams. Recently, other techniques such as synchrotron sources and laser plasma sources have emerged as alternate sources of high intensity X-rays. However, such methods are considerably more expensive and cumbersome in comparison to conventional X-ray tube technologies. Consequently, a high power X-ray anode source is highly desirable.
An X-ray tube usually consists of an anode and an electron-emitting cathode. A small fraction of the electrons bombarding a portion of the anode known as the anode target, cause excitation of target atoms. The energy released during the de-excitation process is sometimes emitted as X-rays. However, most of the energy imparted by electron bombardment is absorbed as heat. The intensity of X-ray production is therefore limited largely by the efficiency of heat dissipation from the anode. Consequently, a large fraction of the research in X-ray tubes has been devoted to schemes of efficiently cooling X-ray anodes by coupling rotation and flow cooling.
Other improvements in increasing the intensity of X-ray sources include anode designs aimed at increasing the efficiency of generated X-rays by using the internal surfaces of a bored anode for the generation and collimation of X-rays (U.S. Pat. No. 4,675,890). In this case electron beams enter one end of the bore and collimated X-rays are generated from the other end. Intensity of generated X-rays can also be further improved by use of X-ray focussing optics.
The major advances in X-ray tube technology have been brought about in the area of efficient rotation (for example, U.S. Pat. Nos. 4,651,336 and 4,608,707) and flow cooling schemes and in efficient use of generated X-rays. Little attention has been paid to the material properties of the anode itself. One proposed scheme relating to anode materials for generation of high intensity carbon X-rays consists of powdered diamond particles embedded in metal/alloys (Japanese patent 55-115024). Alternately it was suggested that thin diamond layers formed on metals could be used for generation of high intensity carbon X-rays (Japanese patent 55-115024). Another scheme proposes using single crystal diamond sources for the production of soft X-rays for high resolution X-ray lithography (J. Appl. Phys. Vol. 49, p 5365-5367). However, these methods have several drawbacks. A shortcoming of diamond sources is the lack of suitable window materials for the efficient transmission of carbon K radiation. Moreover, use of single crystal diamonds is not desirable from the point of view of thermomechanical stresses created at high levels of energy conversion.
The extremely high thermal conductivity of diamond together with low coefficient of thermal expansion and high tensile strength make it extremely attractive as a part of a structure for the effective cooling of X-ray targets. In fact, composite structures based on efficient cooling with the much less conductive graphitized carbon have been suggested in the past. But, until recently, large area diamond crystals were quite expensive. However, the emergence of low pressure CVD diamond technologies make the formation of high quality diamond coatings conforming to specific designs feasible. Additionally, the use of CVD diamond technologies make the formation of single crystalline and polycrystalline diamond coatings attainable. The thermal conductivities of high quality CVD diamond coatings are comparable to that of natural Type IIa diamond (21 W/cm.K at 300 K). This is about five times greater than the thermal conductivity of copper at room temperature. In addition, the thermal conductivity over a wide range of temperatures in artificial diamond may be enhanced by the growth of isotopically pure (.sup.12 C, .sup.13 C or .sup.14 C) single crystalline diamond. The gain in thermal conductivity for isotopically pure diamond (.sup.12 C) is about 50% over that for natural Type IIa diamond at 300 K. The use of different sources of diamond (natural, ultra-high pressure and CVD technologies) gives the possibility for the design and creation of diamond composite structures and devices. Further possibilities exist for synthesis of diamond-non diamond structures and active/passive devices.
The present patnet application describes the synthesis of structures that permit more efficient cooling due to the high thermal conductivity of diamond. These advantages can be incorporated in conjunction with efficient designs for the use of liquid or gas coolants and anode rotation to further improve the high power generation capabilities of the anode. The structures described herein, detail X-ray production from both single and multiple discrete X-ray sources. In addition, some of the proposed structures can perform both the functions of X-ray production and act as vacuum X-ray windows for the transmission of the generated radiation.